An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. Accelerometers are used along with gyroscopes in inertial guidance systems, as well as in many other scientific and engineering systems. One of the most common uses for micro electromechanical system (MEMS) accelerometers is in airbag deployment systems for vehicles. In this capacity, the accelerometers are used to detect the rapid negative acceleration of a vehicle to determine when a collision has occurred and the severity of the collision in order to control deployment of the airbags. Another common use for MEMS accelerometers is in electronic stability control systems, also referred to as vehicle dynamic control, designed to improve a vehicle's handling, particularly at the limits where the driver might lose control of the vehicle.
In certain applications, it may be desirable to employ multiple sensors to detect and measure movement of an object in more than one dimension. To accomplish this task, many prior art devices utilize a cluster of individual packages, each containing a single sensor that detects movement in a particular plane. The multiple axis transducer packages are more complex than their single axis counterparts, which puts pressure on the size, cost, and accuracy of these devices.
One problem that affects the accuracy of transducer packages is that of undesirably high thermal offset. Thermal offset is the non-acceleration induced stress as a function of temperature that is placed on a semiconductor device such as a MEMS device. The temperature coefficient of offset (TCO) is a measure of this non-acceleration induced stress. A large TCO can result in measurement inaccuracies within the MEMS transducer package, thus requiring compensation to reduce the TCO to near zero. Tighter design specifications on the range of allowable thermally induced offset are being called for within the industry to reduce these inaccuracies.
Further increasing the complexity of multiple axis transducer packages is the requirement for accurately measuring movement within different sensing ranges. That is, there is an increasing need for one sensor to detect movement in one sensing range and another sensor to detect movement in a different sensing range within a single multiple axis transducer package. For example, in an airbag deployment system, a first accelerometer of the transducer package may be utilized to detect the rapid deceleration of a vehicle in order to control deployment of the front airbags. A second accelerometer of the transducer package may be utilized to detect side collisions in order to control deployment of the side airbags. For front airbag deployment applications, the sensing range may be a medium-g sensing range of, for example, ten to one hundred g's. In contrast, for side airbag deployment applications, the sensing range may be a high-g sensing range of, for example, greater than one hundred g's. Still other applications call for a low-g sensing range of, for example, less than ten g's. Such an application may be found in vehicle dynamics control.
Accordingly, what is needed is a multiple axis transducer package that is small, inexpensive, and accurate. What is further needed is a multiple axis transducer package that is largely impervious to thermally induced offset and may be readily adapted to detect movement over different sensing ranges along mutually orthogonal axes.